In general, noble metals such as platinum and palladium are used as catalysts for fuel cells and off-gas cleanup. However, the amount of the noble-metal elements on the earth is limited, and therefore this situation requires a reduction of the use of the noble-metal elements. Accordingly, a catalyst using a noble-metal is generally made such that micro powder particles are deposited on the surfaces of carrier particles of, for example, carbon or an inorganic compound. The catalytic action mainly occurs on the surfaces of the noble metals. Therefore, in order to reduce the amount of noble metals used as the catalysts having the structure mentioned above and to maintain sufficient activities as the catalysts, the noble-metal powder on the carrier particle surfaces is required to have a smaller particle size and a larger specific surface area.
Recently, using a metal powder, in particular, noble-metal powder of gold, silver or the like, as pigment of an ink-jet ink has been investigated, for example, in order to do electroconductive-powder printing and metallic printing with ink-jet printers. The noble-metal powder used for such uses is also required to have a smaller particle size and a narrow particle-size distribution, though the requirement is less strict than that for the catalysts.
These types of micro-fine metal powder are produced by high-temperature processing called an impregnation method, a liquid-phase reduction method, or a gas-phase method. In particular, the liquid-phase reduction method, i.e. reducing ions of a metal to be precipitated with a reducing agent in a liquid-phase reaction-system to precipitate the metal as metal powder, has been broadly used thus far because of its ease in simplifying facilities. Various compounds have been proposed as the reducing agent used in the liquid-phase reduction method. Examples of the proposed reducing agents include the following:
Aldehyde (Patent Document 1);
Sodium hypophosphite, potassium hypophosphite, or ammonium hypophosphite (Patent Document 2);
Ascorbic acid (Patent Document 3);
Thiourea or thioacetamide (Patent Document 4);
Ally alcohol, hydrazine hydrate, and hydrogen gas (Patent Document 5);
Propargyl alcohol and hydrogen gas (Patent Document 6);
Alcohol, such as ethanol, and hydrogen gas (Patent Document 7);
Amines, in particular, alkanolamine (Patent Document 8);
Alcohols such as ethanol (Patent Document 9);
Amino-alcohol (Patent Document 10); and
Hydrogen gas or sodium borohydride (Patent Document 11).
In order to produce metal powder having a small particle size by the liquid-phase reduction method, it is effective to conduct the reduction and precipitation of the metal powder at a slower rate. Therefore, it is important to use a reducing agent with lower reducing power. The above-mentioned catalysts are ranked in the order of reducing power from the highest to the lowest as follows: sodium boron hydride or hydrogen gas>hypophosphite reducing agents>ascorbic acid>alkanolamine (amino-alcohol)>alcohol (ethanol, etc.).
Patent Document 1: Japanese Unexamined Patent Application Publication No. 2-169029 (claim 1, page 2, lines 16 to 20 of bottom left column and lines 13 to 19 of bottom right column on same page)
Patent Document 2: Japanese Unexamined Patent Application Publication No. 2-303541 (claim 1, page 2, lines 2 to 7 of bottom left column and from line 18 of bottom right column on page 2 to line 18 of upper left column on page 3)
Patent Document 3: Japanese Unexamined Patent Application Publication No. 4-298238 (claim 1 and paragraph 0005)
Patent Document 4: Japanese Unexamined Patent Application Publication No. 5-161849 (claim 1, and paragraphs 0007, 0009, and 0010)
Patent Document 5: Japanese Unexamined Patent Application Publication No. 6-31166 (claim 1 and paragraph 0005)
Patent Document 6: Japanese Unexamined Patent Application Publication No. 31181 (claim 1 and paragraph 0004)
Patent Document 7: Japanese Unexamined Patent Application Publication No. 8-162122 (claim 1, and paragraphs 0007, 0013, 0014, and 0017)
Patent Document 8: Japanese Unexamined Patent Application Publication No. 11-80647 (claims 14 and 15, and paragraph 0051)
Patent Document 9: Japanese Unexamined Patent Application Publication No. 2000-107606 (claim 1, and paragraphs 0009 and 0017)
Patent Document 10: Japanese Unexamined Patent Application Publication No. 2000-279811 (claim 1 and paragraph 0013)
Patent Document 11: Japanese Unexamined Patent Application Publication No. 2002-231255 (claim 3 and paragraph 0014)